A polymer electrolyte fuel cell generates electricity and heat simultaneously through an electrochemical reaction that occurs on a metal catalyst by supplying a fuel gas such as hydrogen to an anode (fuel electrode) and oxygen in the air to a cathode (oxygen electrode). The polymer electrolyte fuel cell has features such as capability of operating at low temperatures from room temperature up to 100° C., rapid start-up/shutdown, and securing a high output density. For this reason, much attention has been paid to the Polymer electrolyte fuel cell as a potential fuel cell for applications such as a cogeneration system for consumer use, an electric power generator for mobile units such as automobiles, or a portable power source.
Typically, electrodes of a polymer electrolyte fuel cell include a supported catalyst in which a catalyst, such as fine platinum particles, is supported on a support (such as carbon: carbon black, etc.) to form. However, a problem of output reduction arises when operated for a long period of time. The cause for the output reduction is presumed to be due to a decrease in electrochemical surface area of the catalyst caused by leaching of platinum serving as a catalyst, a resistance increase in the polymer electrolyte membrane, and a decrease in electrical conductivity caused by oxidation of carbon serving as a support.
In a polymer electrolyte fuel cell, the catalyst layer of the oxygen electrode is particularly exposed to superacid (protons in the electrode) and a high potential, and as a result, the metal catalyst is oxidized and leaches out. Some of the leached catalyst ions deposit on the surface of undissolved metal catalyst, causing grain growth of the fine catalyst particles. Because they deposit alone without coming into contact with carbon (support) serving as a conductor, they cannot be involved in the reaction in the electrode. This decreases the catalyst's electrochemical surface area (catalyst's effective area) that contributes to the reaction, causing a problem in that the fuel cell output cannot be maintained. Other leached catalyst metal ions diffuse through to the polymer electrolyte membrane, where they deposit, which constitutes a major contributing factor for the increase of membrane resistance.
In order to suppress the aforementioned decrease of catalyst electrochemical surface area, the use of an alloy, such as PtCo, as a metal catalyst has been proposed, and an investigation has been made to improve durability by suppressing the platinum leaching by turning platinum into an alloy (Patent Document 1). Similarly, in order to suppress the leaching of platinum from a catalyst layer to improve durability, the incorporation of a material that forms a planar mononuclear complex with platinum ions in the catalyst layer as a platinum ion capturing agent has been proposed (Patent Document 2). These methods can suppress the decrease of catalyst electrochemical surface area caused by the dissolution of platinum and the diffusion of leached platinum ions into the polymer electrolyte membrane to some extent. However, a sufficient suppression effect has not yet been obtained, and a catalyst layer with a long service life has not yet been achieved.
In order to prevent the problem of carbon serving as a support being exposed to a high potential, and as a result being oxidized and destroyed, the use of carbon with superior corrosion resistance as a catalyst support has been proposed (Patent Document 3). Furthermore, the incorporation of conductive carbon that supports no catalyst in a catalyst layer has also been proposed (Patent Document 4). These methods can suppress the electrical conductivity reduction of the catalyst layer, a decrease in reaction gas diffusibility caused by changes in the pore diameter of the catalyst layer, and so on, but they cannot suppress the dissolution of the catalyst.
Patent Document 1: JP H6-176766A
Patent Document 2: JP 2006-147345A
Patent Document 3: JP 2000-268828A
Patent Document 4: JP 2006-4916A